1. Field of the Invention
This invention relates to cellular multicarrier wireless communication systems.
2. Description of Related Art
With multicarrier modulation, a data signal is split into several lower rate signals, such that the symbol time becomes large compared to the expected multipath delay spread. The subcarrier frequencies may be distributed in different ways. The simplest case is where each mobile terminal has a different but fixed and block of contiguous subcarriers. Other possibilities include hopping of the subcarriers, or the use of a random (non-contiguous set of subcarriers. These latter options have the advantage of reducing the variance of co-channel interference.
An important feature of multicarrier modulation is the cyclic symbol extension (also referred to as guard interval, or guard time). In the downlink, signal components can be delayed by multipath propagation of the signal from the mobile""s dedicated base station. As long as the delay offset of delayed signal components is within the guard time, the multicarrier signal remains undistorted for detection at the mobile terminal. The receiver can make use of the whole energy of all delayed signal components by using non-coherent detection.
In this way, multicarrier modulation avoids the need for complex equalization to remove echoes.
Communication on a downlink or forward link from a base station to mobile terminal or user, can be distinguished into data communication on dedicated data channels, and signaling communication on dedicated signaling channels.
Conventional cellular systems apply different signaling channels in every cell. Thus, for each cell, signaling channels of neighboring cells are either experienced as inter-cell interference, or left unused which reduces spectral efficiency.
Against this background, in accordance with the invention there is provided a cellular multicarrier wireless communication system, in which base stations for a cluster of two or more respective cells transmit the same signaling information in synchronism on the same group of subcarriers, wherein a multiple access preamble is transmitted in each frame to identify the base station from which it is transmitted.
The ability of a multi carrier system to make use of the whole energy of all delayed signal components by using non-coherent detection allows a mobile terminal to receive a composite signal from more than one base station. Hence, multicarrier modulation together with cyclic symbol extension provides macrodiversity.
Ideally, when a signal from a base station is delayed beyond the guard time at the mobile terminal, the signal strength is sufficiently low, due to the distance from the base station, that the mobile station can accurately detect signals from closer base stations where the delay offset is inside the guard time.
The data communication is very different for each mobile terminal, since it carries e.g. voice data, or any other kind of mobile terminal specific data (e.g. multimedia data, image, etc.)
In contrast, signaling communication is not very different for each mobile terminal, since it contains information that is of relevance to all mobiles (e.g. timing reference for initial acquisition, number of mobile terminals in the system, latest billing prices, new software for terminal, time signal, or any other broadcast-like type of information).
The signaling communication also contains user specific data. Thus the signaling channels are used to control the operation of the mobile terminal, e.g. in standby mode the mobile terminal still has to listen to the signaling channel of its respective base station, since the signaling channel is used as a paging channel to wake up the terminal in case of an incoming call.
For the signaling channel the amount of information that is mobile specific, e.g. signaling information regarding the radio link protocol, is of low bit rate nature (incoming call signal, response signal for call request, etc.). That is why it is possible to use one signaling channel throughout the whole cellular system or throughout a smaller cluster of base stations. The number of subcarriers should be chosen according to the expected traffic on the signaling channel and is dependent on the number of users in system.
Preferably, signaling information is transmitted in successive frames each carrying address information identifying mobile stations for which signaling information in the frame is intended.
Signaling information could be addressed to all mobile terminals, to a set of mobile terminals or to individual mobile terminals.
The transmission of frames is preferably prioritized in favor of frames addressed to particular mobile stations.
In order to allow a mobile terminal to distinguish which base stations it is receiving signaling from, a code division multiple access preamble may be transmitted in each frame to identify the base station from which it is transmitted.
Alternatively, a time division multiple access preamble may be transmitted in each frame to identify the base station from which it is transmitted.
The invention also extends to a mobile station for a cellular multicarrier wireless station as claimed in any preceding claim, configured to receive signaling information on a common group of subcarriers within a cluster of two or more cells, in which non coherent detection is utilized in relation to symbols modulated on subcarriers in said group, in which the base station transmitting on the common channel are each identified from a multiple access preamble transmitted in each frame to identify the base station which it is transmitted, and including means to measure the strength of the multiple access signal from each base station.